The aim of experiments in this proposal is to elucidate the intracellular mechanisms (2nd messengers) by which gonadotropin- releasing hormone (GnRH) stimulates expression of the alpha and LH beta subunit mRNAs required for LH synthesis by pituitary gonadotrope cells. The studies will be performed in vitro and physiologic pulses of GnRH used to stimulate pituitary gonadotropes. Gonadotrope responses will be assessed by measurement of GnRH receptors, alpha and LH beta mRNA concentration, LH storage and acute release. The effect of prolactin and gonadal steroids on cellular responses to GnRH will be determined. The intracellular mechanisms involved in transmitting the GnRH signal to stimulate LH subunit gene expression will be examined by measurement of mRNA responses to agents which bypass the GnRH receptor. The effects of both continuous and pulsatile elevation of intracellular calcium and cyclic nucleotides, and constant or intermittent stimulation of protein kinase C on LH subunit mRNA concentrations will be determined. By comparison of the effects of these stimuli with those following GnRH pulses, we aim to elucidate the intracellular mechanism(s) involved in transmission of the signal initiated by GnRH-receptor binding. Subsequent studies will examine if gonadal steroids regulate GnRH action by modifying LH subunit mRNA responses to the second messengers. Other studies will examine the mechanisms involved in desensitization of LH secretion by a continuous GnRH stimulus. The role of mRNA expression and the ability of GnRH to stimulate phosphoinositol hydrolysis in desensitized cells will be examined. The mechanisms of GnRH action have important implications for the regulation of fertility in humans and animals. A pulsatile GnRH stimulus is required to maintain LH secretion and continuous stimulation by GnRH desensitizes LH secretion. Specific syndromes of GnRH deficiency are recognized in humans and fertility can be restored by administration of GnRH in a pulsatile manner. Long acting GnRH agonists which desensitize LH release are being assessed as potential contraceptive agents. The elucidation of the exact mechanisms by which a pulsatile GnRH stimulus maintain LH synthesis and secretion are important for the development of future therapeutic regimens to both enhance or inhibit gonadotropin secretion.